The present invention relates to auditory assistance apparatus. The invention is particularly applicable in hearing aids for the hearing impaired, and is therefore described below with respect to this application.
Hearing aids normally pick up the sound via a microphone transducer, amplify the sound via a low power audio amplifier, and transmit the sound to the user's ear via an acoustic transducer. If a microphone is used, the ambient acoustic noise reduces the signal-to-noise ratio; and when the sound signal is weak (as in the case of a telephone speaker), the noise may actually override and mask the signal so a to make the signal non-detectable.
Some hearing aids feature a magnetic pickup coil, usually termed a "Telecoil", or "T-coil". Such a coil is used for picking up sounds via magnetic induction, typically from a telephone's receiver or from a dynamic speaker having a magnetic core. Another application of such pickup coils is to pick up magnetic induction transmitted by an induction loop, typically installed around the walls in theaters, classrooms, churches, and the like.
The pickup coil is normally connected to the hearing aid audio amplifier and to the output acoustic transducer. The pickup coil of the hearing aid is usually a cylindrical coil of small size and of many windings around a special core. The sensitivity of such pickup coils is not more than 3-4 mV/A/m. The typical T-coil output electrical signal induced by telephone receivers is about 50-100 .mu.V. This signal can only be picked up when the axis of the coil's magnetic core is parallel to the magnetic field's direction, and the pickup coil is situated as close as possible to the telephone receiver or speaker.
Hearing aids including such a pickup coil are especially useful for listening to the telephone since the telephone's receiver produces a very weak sound. Such a coil does not pick up the acoustic ambient noise and thus allows a much better signal-to-noise ratio than a conventional microphone. The improvement in the signal-to-noise ratio permitted by a magnetic pickup coil also results from the fact that the entire attenuation of the magnetic field is at a distance of about 30-40 cm from the sound signal source, while th entire attenuation of the acoustic field of a sound signal source is at a distance of about 30-40 m, a difference of about 1:100.
Magnetic pickup coils, however, are highly sensitive to magnetic noise present in the home, office, street, industrial environments, etc., induced in the audio band. A major source of such ambient magnetic noise is the 50/60 Hz frequency of the AC power line and its harmonics. Other sources include electrical apparatus as transformers, TV and computer monitors, communication lines, electrical motors, and electrical generators, all of which tend to produce low frequency magnetic noise and thereby degrade the potential signal-to-noise ratio of a hearing aid using magnetic pickup coils. Most hearing aids having magnetic pickup coils, therefore, include a high pass filter to attentuate this low frequency noise by cutting off the frequency band below 1 KHz. However, the typical hearing loss is in the high frequencies, and therefore the user of such a hearing aid is denied hearing the lower band frequencies where the user generally has better hearing.
In addition, the magnetic pickup coil must generally be oriented in the direction of minimum noise sensitivity in order to decrease the magnetic noise. However, this orientation may not correspond to the optimal conditions providing an acceptable or optimum level of the desired source signal. The foregoing situation is typical for magnetic loop signal listening and telephone listening.